Speaking at the March of Dimes Celebration of Babies luncheon in Los Angeles on Friday, Megan said: "The whole thing has been overwhelming because I didn't realise you could love something so much, and I know people always say that but I love him so much it hurts, and it's an interesting feeling to have because I never felt that before."
Megan - who married Brian, 39, in June 2010 - also revealed Noah is currently on the verge of laughing.
The 26-year-old actress is sure her heart will "explode" with joy when he lets out his first giggle.
She added to E! Online: "He's getting to that place where he's smiling and he's almost ready to laugh, and I can't wait for that baby laugh that almost makes you feel like you can fly ... like your heart's going to explode."
Megan recently praised her "soul mate" Brian - who also has a 10-year-old son Kassius, with ex-girlfriend Vanessa Marcil - for being such a great hands-on dad with little Noah.
The Hollywood starlet said: "My husband is amazing. He does more work than I do and he's just a champ ... He's so helpful and nurturing and amazing ... He's my soul mate.
"I love [motherhood]. It's what I've wanted to do since I was a little girl."